Conventionally, aromatic polycarboxylic acids have been produced by oxidation of polyalkylbenzene, trimellitic acid from pseudocumene, trimesic acid from mesitylene, pyromellitic acid from durene and mellophanic acid from isodurene are known.
In the oxidation of akylbenzenes correspond to aromatic polycarboxylic acids, the alkylbenzenes differ from one another in reactivity depending upon the position of substituted methyl group. Trimellitic acid or pyromellitic acid producd from pseudocumene or durene has a structure in which one carboxylic acid is positioned in opposition to another, resulting in reduced activity of the catalyst activity and a decrease in yield from the oxidation as compared with polymethylbenzenes not having such a structure.
Therefore various proposals have been made for the improvement of the catalyst system. For example, JP-A-Hei-2-184652(1990) discloses a method in which catalyst is charged in two steps in a batch system in the liquid oxidation of durene to pyromellitic acid using Co—Mn—Br catalyst.
Although yield of the product is increased by this improvement of the catalyst, this system is complicated and this method is not applicable to continuous or semi continuous-systems because the pyromellitic acid produced decreases the activity of the catalyst.
Polycarboxylic aromatic aldehydes are known as raw materials for producing aromatic polycarboxylic acids. JP-A-Sho-57-38745(1982) discloses a method in which a polycarboxylic aromatic aldehyde is oxidized to produce an aromatic polycarboxylic acid in the presence of cobalt, manganese and bromine in acetic acid solvent. JP-B-Hei-7-116097(1995) discloses a method in which a polycarboxylic aromatic aldehyde is oxidized in the presence of iron, manganese and bromine in water solvent, to produce pyromellic acid.
The catalyst of cobalt, manganese and bromine using polycarboxylic aromatic aldehyde as raw material in JP-Sho-57-38745(1982), results in necessity of the improvement of reaction rate and yield. The problem of corrosion may occur in JP-B-Hei-116097(1995) because the concentration of bromine is high.